send-invisible

Type a password in a telnet or process buffer.

This command prompts for a line of text, hiding what you type with
*'s, and then sends it to the telnet or concurrent process running
in the current buffer. It arranges things so the password isn't
recorded for show-last-keys or in a command history. A numeric
prefix argument makes the command omit the newline sequence it
includes by default after the line of text you enter.